A lot of people are not able to extract the ROMFS and get the editor to work. So, I will explain it here! The best part of this tutorial is that you do not even need a 3DS (if you have your ROM ready)!

Warning: Do not use any other versions (including newer ones) of HackingToolKit3DS, or you cannot get the RomFS extracted properly!

The game can be in any region, but it must be decrypted. You may follow the tutorial over here for decrypting the game.
_________________________Check out these awesome hacks! Beyond DS is now my major hack!

By now you can easily and especially legally dump the cxi file of NSMB2 from your 3ds cart or downloaded title via godmode9. These are easy to extract and rebuild and can even be ran on Citra.
_________________________GitHub - Kuribo64 - YouTube